Process for the production of diffusion resistant coatings



April 26, 1960 J. SCHUTT 2,934,456

PROCESS FOR 'rmz PRODUCTION OF DIFFUSION RESISTANT commas Filed March25, 1957 b/$-(4hyd/'oxy-phenyl)-Z2 pmpane- -epichlorhydrlh reactionpmducf fused alum/mum sprayed onto the metal base metal bake,.randb/asled INV ENT OR.

JAS'QIZJLQZE United States Patent C PROCESS FOR THE PRODUCTION OFDIFFUSION RESISTANT COATINGS The invention relates to a process for theproduction of protective coatings or covers for supports or members ofall kinds, and particularly to the production of anti.- corrosionprotective coatings for metallic supports and members.

Many different materials have already been proposed for use asprotective coatings, such as metals of lowmelting point, varnishes,plastics or synthetic resins and the like. The recently evolvedso-calledepoxide resins, which are products of reaction betweenbis(4-hydroxyphenyl)2,2-propane (diphenylol-propane) and epichlorhydrinand having two terminal epoxide groups, are of particular interest ascovering materials, as this kind of plastic or synthetic resin exhibitsa high degree of resistance to chemicals and corrosive liquids, inparticular to sea water, and also exhibits a high resistance to heat. Byvirtue of these properties the epoxide resins are particularly suitablefor use as coating materials for ships, steam kettles, water kettles aswell as for hot water pipes, central heating equipment and the like,which fields are not covered by other artificial or synthetic materials.

The general use of epoxide resins of this kind for the given technicalrange of application has, however, been confronted with the ditlicultycaused by their limited resistance to diffusion. It has repeatedly beenshown that the adhesion of epoxide layers on supports, as for examplesheet iron, sheet aluminum or the like common production materials, issufficient or inadequate in the presence of water, so that theadvantageous properties of the epoxide resins are not displayed toadvantage because of the early detaching or loosening of the coating mmthe support which is explained by the fact that water diffuses throughthe coating.

It is an object of the present invention to avoid these difiiculties andto make it possible to produce coatings based on bis(4hydroxyphenyl)2,2'-propane-epichlorhydrin reaction products comprisingtwo terminal epoxide groups which adhere strongly to supports of allkinds,

particularly to metallic supports, which coatings are particularlydistinguished by their dilfusion resistance.

It has been found that the crystalline structure of the surface of thesupport is an essential factor in the adher-ence anddiffusion-resistance of coatings based on bis(4hydroxyphenyl)2,2-propane-epichlorhydrin reaction products having twoterminal epoxide groups. The

providing an adhesion layer or film of the said kind on the surface ofthe support, body or member to which the 2,934,456 Patented Apr. 26,1960 epoxide resin is to be applied. The adhesion layer or film may berelatively thick, for example 0.2 mm; it may also be very thin, forexample, it may be only a monomolecular layer.

Many different ways are known for producing a layer having a spinelcrystal structure. Generally, spinels may be of the general compositionindicated by the formula A 130, in which the symbols A and B signifymetals which are tervalent and bivalent respectively and the symbol 0represents oxygen. Examples of the spinels are the aluminium spinels Al(BO particularly 2( 4), 2( 4), 2( 4), A 2 4). Al (CuO and the like, theiron spinels Fe (BO such as Fe (ZnO Fe (MnO Fe (CuO Fe (NiO Fe (CoO andthe like, the chromium spinels Cr (BO such as Cr (MgO Cr (ZnO Cr (FeOand the like. The metals A and B given in the general formula for thespinels may be different metals. as illustrated in the above examples;they may also indicate the same metal if this metal can be bivalent aswell as tervalent. A particularly important example for the purposes ofthe invention is Fe (FeO All of the given examples of spinels have alattice constant of approximately from 8 to 8.5. The lattice constantsof some of the spinels hereinbefore referred to are given in thefollowing table:

There are also known spinels of simple oxide composition, for example,-Fe O having a lattice constant of 8.40 and 'y'Al O having the latticeconstant of 7.90. Synoptical tables of these and other known spinels aregiven by DAns-Lax in Taschenbuch fiir Chemiker und Physiker 1953, page175.

An advantageous method for producing an adhesion layer on a supportaccording to the invention consists in spraying a metal in liquid formon to the support in the presence of oxygen, whereby the liquid metalforms with oxygen, oxides of the spinel-crystal type. Thus, according tothe invention, liquid aluminium may be sprayed on to any support, thealuminium forming together with atmospheric oxygen 'y-Al O The adhesionlayer so formed is of spinel character and is suitable for theapplication thereon of diffusion-resistant, epoxide resin layers.

It is also possible to form iron oxide adhesion layers instead ofaluminium oxide adhesion layers by applying molten iron to the surfaceof the support in which case the iron reacts with oxygen to form -Fe Owhich has a spinel structure. However, this method can only be employedwhen the support used is not adversely alfected by the heat ortemperature of the liquid iron which is sprayed thereon. Generally,aluminium will be preferred in view of its low melting temperature andthe simplicity of the means or apparatus for the spraying ormetallization.

However, it will be understood that other metals may be usedto form,according to the process of spraying or metallization, oxide adhesionlayers of spinel crystalline structure. Thus, for example, other thanaluminium it? self, difierentaluminium' alloys, such asaluminium-magnesium, aluminium-zinc and aluminium-manganese, as

well as iron and manganese, may be used in the manner hereinbeforedescribed.

Adhesion layers of spinel crystalline structure may also be produced bypickling or blanching metallic supports, such as iron and aluminium,with oxidising liquids, particularly nitric acid. The pickling processmay be carried out either in the hot or in the cold; suitableconcentrations are from 0.1 to 2 moles HN per litre. The duration of thepickling process may be approximately 1 hour when caried out in thecold, whilst in the hot the duration of the pickling process maybe fromto minutes, until spinel'layers are formed which are suitable asadhesion layers. The formation of the desired spinels with a latticeconstant of approximately from 8 to 8.5 can be assured by the additionof particular metal nitrates to the pickling solution. However, it is tobe observed that in the pickling treatment no metallic salt is to beadded, which is, in the pickling solution, more electropositive than themetal support; otherwise a galvanic metalic precipitate or deposit wouldbe formed on the metal support which would prevent or hinder theformation of a spine] adhesion layer. If necessary or desired, in such acase an electrical inverse or counter potential or voltage may beapplied to the metal support to prevent or hinder such formation of agalvanic precipitate.

If a spinel adhesion layer is to be produced on ironby pickling, thebest method of carrying it out is to use nitric acid of the givenconcentration of from 0.1 to 2 moles: per litre and to add zinc nitrateto the pickling solution, advantageously in a ratio of 2 moles HNO to 1mole Zn(NO whereby the iron spinel Fe (ZnO is formed on the surface ofthe iron, the iron spinel having a lattice constant of 8.40. Instead ofzinc nitrate another metal nitrate may be used which is capable ofreacting with iron to form iron spinels, such, for example, as manganesenitrate, magnesium nitrate and the like. In this way it is readilypossible to adjust or control the lattice constant of the said spinellayer. It has been found that the lattice constant of the spinel to beproduced should correspond as nearly as possible with the distance ofthe polar phenol ether bonding of the bis('4-hydroxyphenyl)'-2,2-propanemolecules, in order to obtain'the optimum diffusion-resistance.

The pickling treatment is particularly advantageous for aluminium andaluminium alloy supports. With iron and steel supports, the picklingtreatment is mainly applied in the case of cold-rolled sheets andoxide-free steels which are free from magnetic oxide (magnetite)inclusions. In othe rcases, iron and steel supports are in generalpreferably treated by spraying or metallization with molten aluminium.

After the pickling or like treatment, the treated surfaces are wellwashed with water and then dried, whereupon the epoxide resin coatingsare applied in known manner.

The layers or coatings of epoxide resin which, according to theinvention, can be applied on thesupportsprovided with an adhesion layer,are condensation reaction products of bis(4-hydroxyphenyl)-2,2-propanewith epichlorhydrin. Such resins are available commercially; forexample, they are sold by the Shell Company under the trade name ofEpikotef. They are soluble in organic solvents such for example, asglycollic ether (Glykolaether), ketones, mixtures of aromatichydrocarbons and alcohols, and, after addition of known crosslinkingagents, such as organic amines, condensation of formaldehyde with urea,phenol and the like, they are. applied on the adhesion layer provided inaccordance with the invention by brushing or spraying and then hardenedat room or elevated temperature. 7

The. invention is. illustrated in the following examples:

Example 1 ing. With the use of. a'propane-oxygen combustion gas .layerso produced, four coatings of an air-drying and amine-hardening epoxideresin varnish were applied. After an uninterrupted period of operationof six months at boiling temperature, the boiler showed no evidencecorrosion.

For the purposes of comparison, a boiler of the same kind was cleaned bysandblasting and, without the formation of an adhesion layer, it wascoated in precisely the same way with the epoxide resin varnish. Thetest in boiling water showed a resistance of only 14 days. After sixweeks, the boiler'was completely useless due to the numerous areas ofcorrosion and loosening and/ or peeling of the coating.

The superiority of a coating on a spinel crystalline adhesion layerprovided in accordance with the invention over a coating of the samekind but without an adhesion layer, can also be shown by electricalmeasurement or" the insulation resistance of the coatings. In thepresent case, the container or boiler on the surface of which theadhesion layer was produced and over which the epoxide resin varnish wasapplied, had'an insulation value or resistance of 3.10 M9 measured witha Teraohmrneter, with a testing electrode of 100 mm. and a layerthickness of the resin coating of an average of 0.15 mm. The insulationvalue or resistance was unaltered after boiling for a period of 8 hours.

0n the other hand, the same layer thickness of the same coating on asheet or plate of the same kind, but applied without a spinel layer,showed even after a boiling time of 5 minutes a decrease of theinsulation value or resistance to 3.10 M9, after 10 minutes a furtherdecrease to 2-3.10 Mt), after 15 minutes yet a further decrease to 2.10MO and after 30 minutes boiling time the insulation value or resistancewas below 2.10 M52.

In the drawing is disclosed a product produced in accordance withExample 1. The word legends under the drawing are self-explanatory.

Example 2 cover or coating of extraordinary resistance to boiling.

Example 3 The surface of a container made of cold-rolled iron sheet was,in the manner described in Example 2, prepared and pickled by l-nonnalHNO without any further additions. A surface layer consisting of Fe fiefl having a lattice constant of 8.41 was thereby formed, which ensuredthat the epoxide resin varnish applied thereon had an outstandingdiffusion-resistance.

Example 4 A container made of aluminium was degreased, filled with waterto which was afterwards added that quantity of concentrated nitric acidrequired to give a 1% solution. The container was heated and itscontents brought to the boil. After one hour the container was emptied,thoroughly" washed and'dried. The inner surface of the container wascoated with a very thin layer of aluminium oxide having a spinelcrystalline structure. Upon the adhesion layer so formed, an air-dryingand amine-hardening epoxide resin varnish was applied. After boiling forweeks, the surface layer did not show any defects. In comparison, twoaluminum containers, one of which had an anodically oxidated surface,were coated with the same epoxide resin varnish without the previousformation of a spinel adhesion layer. Even after boiling for only 15-30minutes the coatings had begun to peel or separate.

Example An aluminium sheet, after being degreased was treated for onehour in the cold with a pickling solution which contained 1 mole END-and /2 moleMg(NO per litre. An adhesion layer of Al (MgO having a spinelcrystalline structure and a lattice constant of 8.09, was therebyproduced. When Zn(NO and Mn(NO were each used separately instead of theMg(NO in the pickling solution in an amount of /2 mole per litre, thespinels Al (ZnO with a lattice constant of 8.10 and Al (MnO with alattice constant of 8.26 were produced on the aluminium sheetsrespectively. All of these layers constitute outstanding adhesionlayers, upon which epoxide resin varnishes exhibit extraordinarilydiffusionresistance.

The coatings or coverings according to the invention can be producedupon any kind of support. The production of an adhesion layer byspraying or metallization with liquid muminium may be carried out onsupports of, for example, concrete, walling, brickwork, wood or metal.The production of the adhesion layer by pickling or treatment with anoxidising liquid, is, however, only possible with metallic supportingmaterials.

What I claim is:

1. A process for the production on a metallic support of adiffusion-resistant protective coating, based on an epoxide resin havingtwo terminal epoxide groups, which comprises providing the metallicsupport with an oxidic adhesion layer having a spinel-like crystallinestructure with a lattice constant of approximately from 8 to 8.5, andapplying on the adhesion layer a coating of abis(4-hydroxyphenyl)-2,2-propane-epichlorhydrin reaction product.

2. A process for the production of a protective coating on a metallicsurface which comprises spraying on to the metallic surface in thepresence of oxygen a liquid metal being capable of forming an oxidicadhesion layer having a spinel-like crystalline structure having alattice constant of approximately from 8 to 8.5 and applying on saidlayer a coating of a bis(4-hydroxyphenyl)-2,2'- propane-epichlorhydrinreaction product.

3. A process according to claim 2, in which the metal sprayed on to themetallic surface is aluminium and the layer formed thereby consistsessentially of 'y-Al O 4. A process according to claim 2, in which themetal sprayed on to the metallic surface is iron and the layer formedthereby consists essentially of -Fe O 5. A process for the production ofa protective coating on a metallic support which comprises pickling thesurface of the support with diluted nitric acid to form on to saidsurface an oxidic adhesion layer having a spinel-like crystallinestructure with a lattice constant of approximately from 8 to 8.5 andapplying on said layer a coating of abis(4-hydroxyphenyl)-2,2-propane-epichlorhydrin reaction product.

6. A process according to claim 5 in which a support consistingessentially of an iron alloy is used.

7. A process according to claim 5, in which the pickling liquor containsa salt of a metal selected from the groups of metals being capable toform with the metal of the support an oxidic adhesion layer of the typeA BO wherein the symbols A and B represent difierent metals and one ofwhich represents also the metal of the support, said layer having aspinel-like crystalline structure with a lattice constant ofapproximately from 8 to 8.5.

8. A process according to claim 7, in which the metallic supportconsists essentially of iron and the pickling liquor contains dilutednitric acid and Zn(NO 9. A process according to claim 7, in which themetallic support consists essentially of aluminium and the picklingliquor contains diluted nitric acid and a nitrate selected from thegroup consisting of Zn(NO and MI1(N03)2 and Mg(NO References Cited inthe file of this patent UNITED STATES PATENTS 2,171,545 Edwards et a1.Sept. 5, 1939 2,487,137 Hoover et a1. Nov. 8, 1949 2,662,034 Mason eta1. Dec. 8, 1953 2,703,765 Osdal Mar. 8, 1955 2,707,691 Wheildon May 3,1955 2,748,030 Silversher et a1 May 29, 1956 2,762,724 Brennan Sept. 11,1956 FOREIGN PATENTS 655,891 Great Britain Aug. 8, 1951

1. A PROCESS FOR THE PRODUCTION ON A METALLIC SUPPORT OF ADIFFUSION-RESISTANT PROTECTIVE COATING, BASED ON AN EPOXIDE RESIN HAVINGTWO TERMINAL EPOXIDE GROUPS, WHICH COMPRISES PROVIDING THE METALLICSUPPORT WITH AN OXIDIC ADHESION LAYER HAVING A SPINEL-LIKE CRYSTALLINESTRUCTURE WITH A LATTICE CONSTNT OF APPROXIMATELY FROM 8 TO 8.5, ANDAPPLYING ON THE ADHESION LAYER A COATING OF ABIS(4-HYDROXPHENYL)-2,2''-PROPANE-EPICHLORHYDRIN REACTION PRODUCT.